The present invention relates generally to a differential amplifier, and more particularly to a high switching speed differential amplifier and the amplification method thereof.
In the driver circuit design for laser diodes, it is required a gain stage capable of providing large current driving and high switching speed. FIG. 1 shows a basic scheme 10 of a driver circuit for laser diodes, which includes a front-end gain stage 14 and a modulation stage 16. The gain stage 14 is inputted a pseudo emitter coupling logic (PECL) signal 12, and its output is connected to the modulation stage 16. For the large capacitive delay effect in the modulation stage 16, the gain stage 14 is employed to amplify the pseudo emitter coupling logic signal 12 to improve the system.
Current gain stage 14 usually utilizes traditional differential amplifier. FIG. 2 shows a traditional differential amplifier 34, which comprises three parts, a first and a second passive loads 70a and 70b, a differential pair 72, and a current source 74. The first and second passive loads 70a and 70b are constructed with resistors 18 and 20 connected to a first and a second supply voltages 46 and 48, respectively. The differential pair 72 is composed of a pair of NMOS transistors 26 and 28 with their gates to receive a pair of differential input signals 36 and 38, respectively, also shown by Vin+ and Vinxe2x88x92 in FIG. 2. The outputs of the NMOS transistors 26 and 28 are connected to the resistors 18 and 20 through nodes 22 and 24, respectively. The current source 74 is composed of an NMOS transistor 20 with its gate biased, source connected to ground 32, and drain connected to the differential pair 72. The operation of the differential amplifier 34 is briefly described in the following. Once the gates of the NMOS transistors 26 and 28 receive the differential voltage signals 36 and 38, the differential pair 72 transforms them to a current flowing through the resistor 18 from the drain current of the NMOS transistor 26. Resulted from the small signal module, it is obtained a voltage gain Gmxc3x97Rd, where Gm is the transconductance of the differential pair 72 and Rd is the resistance of the resistor 18. To improve the switching speed of the differential amplifier 34, the resistance Rd of the resistor 18 must be low, while that also reduces the voltage gain Gmxc3x97Rd at the same time. If the resistance Rd of the resistor 18 is increased to obtain larger voltage gain Gmxc3x97Rd, the larger resistor 18 will lower the frequency response and slow down the switching speed. In other words, the traditional differential amplifier 34 cannot provide high voltage gain and high switching speed at the same time. Hence, it is desired a differential amplifier with high amplification gain and high switching speed.
Accordingly, one object of the present invention is to disclose a differential amplifier to provide high gain and high switching speed at the same time.
In a differential amplifier, according to the present invention, there is included a differential pair, a current source connected to the differential pair, and two active loads connected to the differential pair. The differential pair is composed of a pair of MOS transistors to receive a pair of differential signals from two inputs. Each of the active loads includes a pair of MOS transistors, among which one active load is connected to an output of one MOS transistor in the differential pair and forms two paths switched therebetween in response to one of the inputs of the differential pair, and another active load is connected to an output of the other MOS transistor in the differential pair and forms third and fourth paths switched therebetween in response to another input of the differential pair.